1. Field of the Invention
The present invention relates to superconducting oxide thin film device having a superconducting oxide thin film formed on a dielectric substrate.
2. Background of the Invention
A yttrium-based superconducting oxide thin film (hereinafter referred to as a "YBCO thin film") which has a high critical current density and a high critical temperature, is considered useful as a material for making Josephson junction devices and high-frequency passive devices.
Also, LaAlO.sub.3 generally has a high relative dielectric constant coupled with a low dielectric loss tangent, and this has heretofore provided satisfactory results when used as a high-frequency dielectric material.
Therefore, it is possible that a device composed of a YBCO thin film and a monocrystalline LaAlO.sub.3 substrate may have significantly improved device characteristics at an operating frequency of several gigahertz.
Although when a YBCO thin film is directly formed on a monocrystalline LaAlO.sub.3 substrate, the YBCO thin film does grow heteroepitaxially to give a monocrystalline film thereon, this technique is problematic in that the adhesion between the film and the substrate is poor and Ba in the YBCO film diffuses into the substrate during the formation of the film on the substrate.
The poor adhesion between the substrate and the YBCO thin film causes peeling of the YBCO thin film from the substrate during production of the device. In addition, it lowers the reliability of the device produced.
The diffusion of Ba into the substrate results in the deficiency of Ba elements in the vicinity of the interface of the YBCO film, thereby degrading the performance characteristics of the device due to crystal defects in the film (for example, to lower the critical temperature of the device or to lower the critical current density thereof), which, in a worst-case scenario, impairs the superconductive condition of the device due to the variation in the crystal structure of the film formed on the substrate.